Everything succeeded, but my PC did not come up, after RMA they sent it back becuse "it works for them". I put everything back togherther and IT DOES NOT WORK. Technical on the phone was not able to help me.

Can someone help me out to identify the problem. I think that the new BIOS no longer supports my i3 CPU or something like that, assuming they don't lie to me. But even if I confirm it works with different CPU, I will need a BIOS downgrade and that can be done only by ASUS RMA.

Well, your assertion *is* possible; they could have inserted some restriction like this in the new BIOS -- but the only way to be sure is for them to test with another 6th gen. i3 processor (which they likely didn't do). Make them.